The present invention relates to direct current motor structure, more particularly to a motor having a permanent magnet stator and a rotor comprising a plurality of isolated sections of magnetizable material, each bearing windings that are energized independently of other rotor winding sections in accordance with rotor position with respect to the stator.
Direct current motors have versatility in a wide range of applications. The availability of a battery power source for dc motor equipped devices facilitates a portability aspect that is not readily available for a-c motor drives. As development of the battery has progressed, the challenge of providing efficient direct current motor drives for vehicles, as a viable alternative to combustion engines, has become more compelling.
U.S. Pat. No. 5,164,623 to Shkondin describes mounting of a motor on the wheel of a vehicle for directly driving the vehicle. Shkondin contemplates application of such an arrangement to electric cars, bicycles, wheelchairs and the like. U.S. Pat. No. 4,754,207 to Heidelberg et al. recognizes a variety of applications in which electronically controlled motors have become increasingly important.
Heidelberg et al. provide a dc motor having a rotor comprising a continuous ring of a plurality of permanent magnets successively alternating in polarity. The stator comprises a plurality of individually switchable electromagnet poles disposed to be circumferentially separated from the rotor magnets by a radial air gap. Several adjacent electromagnets form a phase group. The radially inward base portions of adjacent electromagnet poles in each group are in surface area contact with each other to form a magnetic flux path. The electromagnetic circuit is broken at the transition points between adjacent groups of electromagnets. Sensors detect relative rotational position between rotor and stator elements to control electronic switching of the individual electromagnet windings. Electromagnets belonging to a common group are switched simultaneously with one common electronic switching means per group. Windings of the electromagnets in adjacent groups are different phases and are switched at different times.
Of concern in implementation of stator winding switched energization is the avoidance of unfavorable consequences such as rotation irregularities. For example, simultaneous switching of all motor phase windings can cause pulsating output torque. Alleviation of these effects, with varying success, can be obtained by appropriately switching all phases at different times or by simultaneously switching certain winding combinations that are distributed symmetrically about the stator periphery and bear certain positional relationships with the permanent magnet poles of the rotor. However, switching of adjacent windings at different times leads to detrimental effects if the windings are linked to a continuous magnetic circuit path. Heidelberg et al. addresses this problem by grouping stator poles in separate magnetic circuit paths. However, such arrangement has an irregular stator pole geometry that requires some poles being wound differently from others. This configuration complicates the manufacturing process and compromises motor efficiency.
The need remains for an improved motor amenable to simplified manufacture and capable of efficient flexible operating characteristics.
The present invention fulfills the above-described needs, at least in part, in the provision of a rotary electric motor in which a stator comprises a first number of permanent magnet poles substantially similar to each other in configuration and spaced substantially equidistantly with alternating polarity along a radial air gap, and a rotor that comprises a second number of salient poles distributed along the radial air gap, the salient poles divided in groups that have magnetic circuits isolated from each other. Although specific numbers of stator and rotor poles are illustrated, the invention is applicable to other embodiments having different numbers of stator and rotor poles. The rotor pole groups are distributed equidistantly along the radial air gap. Windings of each group of salient poles are individually switchably energized in accordance with relative position between the stator and rotor so that magnetic polarities of the salient poles are changed to provide rotation. The switching may be performed by a mechanical commutator or by electronic switches activated in response to position sensing signals.
One aspect of the invention is that all of the rotor salient poles are of substantially the same dimension. The magnetic paths between pole groups are discontinuous. Each group of salient poles that forms a complete magnetic circuit path may comprise two adjacent poles with windings arranged, when energized, for effecting opposite magnetic polarity for the two adjacent poles. When switchably energized, current in the opposite direction in the windings effects a reversal of the magnetic polarities. Alternatively, each isolated magnetic circuit rotor group may comprise a number of salient poles other than two, as long as all adjacent poles when magnetized are of opposite polarity. For example, each salient pole can form a group isolated from the others. Switching energization for all of the windings is appropriately timed to effect smooth operation.
Preferably, the rotor salient pole faces and the stator permanent magnet pole faces are of the same radial dimension. The radial distance between salient poles of each rotor group is the same for all groups and substantially the same as the radial distance between stator poles. The rotor pole groups are distributed equidistantly about the radial air gap. As the total number of rotor salient poles need not be equal to the total number of stator permanent magnet poles and the radial distance between adjacent poles of different groups is constant, this distance need not be the same as the distance between poles of the same group.
In a preferred embodiment, the direct current motor has an inner stator and outer rotor that radially surrounds the stator. Alternatively, the outer rotor may comprise a permanent magnet annular ring with the switched salient pole windings configured on the inner stator. In the latter case, the stator poles would form groups of isolated magnetic circuits.
The present invention thus has advantages of simplified manufacture, as the salient poles can be uniformly wound, and the pole sizes can be uniform for both stator and rotor. In addition, efficient and smooth motor operation can be obtained.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the invention is shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.